An electric double layer capacitor that can be repeatedly used by charging is a capacitor in which a charge is stored in an ion adsorption layer that is formed in pores of a porous carbon electrode, such as an activated carbon, that is, an electric double layer. Since this electric double layer capacitor has a long lifetime and high output, it has been used as a power source for backing up of computer memory. Recently, attention has been rapidly paid to the electric double layer capacitor as an electric power storage system mounted on a railway vehicle and an auxiliary power source for a hybrid vehicle.
In recent years, in order to improve the energy density of the electric double layer capacitor, hybrid capacitors in which an activated carbon electrode and an active material for a rechargeable battery are used for an electrode material have been developed. A lithium-ion capacitor is one of the hybrid capacitors. In this lithium-ion capacitor, an activated carbon, a carbon material for a lithium-ion battery negative electrode, and an organic electrolyte solution for a lithium-ion battery are used for a positive electrode, a negative electrode, and an electrolyte solution, respectively.
As shown in FIG. 17, a power source 14 is connected between two activated carbon electrodes 12 and 13 that are immersed in an electrolyte solution 11 and an electric double layer capacitor 10 is charged by application of voltage. During charging, electrolyte ions are adsorbed on surfaces of the electrodes. Specifically, anions (−) in the electrolyte solution 11 and cations (+) in the electrolyte solution 11 are attracted to holes (h+) in the positive electrode 12 and electrons (e−) in the negative electrode 13, respectively, and the holes (h+) and the anions (−) and the electrons (e−) and the cations (+) are arranged at a minimum distance of several angstroms to form an electric double layer. This state is maintained even when the power source is taken off. A power storage state is maintained without use of a chemical reaction. During discharging, the adsorbed cations and anions are each detached from the electrodes. Specifically, the electrons (e−) return to the positive electrode 12, resulting in a decrease in the holes (h+). As a result, the anions and the cations are diffused in the electrolyte solution again. Thus, materials for the capacitor are not changed over the whole process of charging and discharging. Therefore, a long lifetime can be kept without generation of heat and degradation due to a chemical reaction.
The electric double layer capacitor is characterized in that (1) charging and discharging can be carried out at high rate, (2) the reversibility of charge and discharge cycle is high, (3) the cycle life is long, and (4) it is environmentally friend since heavy metal is not used for an electrode and an electrolyte. This is because heavy metal is not used for the electric double layer capacitor, and the electric double layer capacitor is operated by physical absorption and detachment of ions and an electron transfer reaction of chemical species is not caused.
Since energy (E) stored in the electric double layer capacitor is proportional to a product of a square of charging voltage (V) and an electric double layer capacitance (C) (E=CV2/2), an increase in the capacitance and the charging voltage is effective for the improvement of an energy density. The charging voltage of the electric double layer capacitor is usually suppressed to about 2.5 V. A reason for this is described as follows. When the electric double layer capacitor is charged at a voltage of 3 V or more, electrolysis of the electrode and the electrolyte solution starts, and as a result, the capacitance decreases and the electric double layer capacitor is deteriorated.
At present, as shown in FIG. 18, a practical activated carbon for an electrode of an electric double layer capacitor is manufactured by adding an appropriate amount of conductive auxiliary agent such as carbon black to activated carbon particles having a size of 1 to 10 μm, molding the mixture using a fibrillated binder such as a polytetrafluoroethylene-based material into a sheet. It may be considered that not only the activated carbon and the electrolytic solution, but also the binder and the conductive auxiliary agent that configures the activated carbon for an electrode affect the decrease in the capacitance due to charging of this electric double layer capacitor at a voltage of 3 V or more.
In order to further increase the capacitance of the electric double layer capacitor, which is not the purpose of solving the decrease in the capacitance, an activated carbon for an electrode that does not contain a binder and a conductive material, that is, a seamless (without a seam) activated carbon for an electrode in which a contact interface is not present between activated carbon particles has been proposed up to now (for example, see Non-Patent Document 1). In Non-Patent Document 1, an activated carbon for an electrode is directly manufactured using a feature of a sol-gel method that is excellent in moldability without use of a binder. When the capacitance of an electrode (binder-free electrode) using the activated carbon that is manufactured without use of a binder is higher than that of an electrode using an activated carbon that is manufactured using a binder and the thickness of the activated carbon for an electrode is large, it is confirmed that differences thereof are remarkable.
As a material concerning another seamless activated carbon for an electrode, a carbon material that has a fine porous structure produced by firing a polyacrylonitrile-based polymer (PAN) porous material and a specific surface area of about 1,000 m2/g has been disclosed (for example, see Non-Patent Document 2). The activated carbon for an electrode shown in Non-Patent Document 2 is manufactured by dissolving PAN in a mixed solvent of dimethyl sulfoxide and water under heating with stirring, and heating a cooled molded body at 230° C. for 1 hour in air, followed by heating at 900° C. for 2 hours in a carbon dioxide/argon atmosphere.
As another manufacturing method, a method for molding a tablet-shaped carbon material without a binder has been disclosed (for example, see Patent Document 1). In Patent Document 1, a phenol compound reacts an aldehyde compound in the presence of water and a catalyst in a disc-shaped container to obtain a tablet-shaped wet gel, water in the wet gel is substituted with a hydrophilic organic solvent and freeze-dried to obtain a tablet-shaped dry gel, and the tablet-shaped dry gel is fired in an inert atmosphere to produce a tablet-shaped carbon material. The tablet-shaped carbon material molded without a binder by this method has a microstructure such as micropores that are fine pores with a diameter of less than 2 nm and mesopores (fine pores with a diameter of 2 to 50 nm).
Further, a block of a carbonized resin porous material that has continuous pores inside and is activated has been disclosed (for example, see Patent Document 2). Herein, an activated carbon block obtained by carbonization of a phenolic resin molded body followed by activation is shown as a preferable example.